The present invention relates generally to methods and apparatus for measuring fluid flow in a conduit and more especially to such methods and apparatus requiring operation over a wide range of flow rates.
Current flow sensors for fuel measurement in aircraft engines are both expensive and require frequent replacement. These flow sensors are required to operate on fluid flows varying from no flow, to a few hundred pounds per hour (pph), to 5000 pph or more.
Cantilevered beam flow meters are known and provide adequate measures of fluid flow in many applications, however, the prior art designs are often inadequate for applications having a wide range of fluid flow rates. The force applied on the cantilever beam is governed by: the area of the beam exposed to the flow; the drag coefficient which is determined by the sensor and the pipe geometries as well and the flow characteristics; specific gravity of the fluid being measured; and the square of the flow velocity. Because of this square relationship, the drag force on the beam is far greater at high flows compared to low flows. As noted earlier, in typical aircraft applications the operating fluid flow can vary from a few tens of pounds per hour (pph), for example, during descent or idle operation to thousands pph or more, for example, during take-off. The cantilever beam, therefore, has to be designed to operate and survive the high flow which results in a very large drag force. Moreover, the sensitivity of the device is as important at the low flows (very small drag force) as well as high flows (unproportionately large drag force).
It is desirable to provide a flow sensor exhibiting a reduced likelihood of damage at high flow rates and enhanced sensitivity at low flow rates. It is also desirable to provide a highly reliable flow sensor at low cost.
The present invention provides solutions to the above problems by providing a cantilever beam sensor in which the drag area of the beam decreases as the flow increases. This allows the flow meter to operate accurately both at low and high flows (high turndown ratio). The present invention further provides a flow sensor based on thick film technology with a wide dynamic range.
In accordance with one form the invention, the dynamic range of a fluid flow sensing device of the type that provides a measure of fluid flow based on the force the flowing fluid exerts on a surface of a sensor member exposed to that fluid flow, is enhanced by reducing the effective area of the sensor member surface which is exposed to the fluid flow as the fluid flow rate increases, and increasing the effective area of the sensor member surface which is exposed to the fluid flow as the fluid flow rate decreases. The effective area may be increased and decreased by changing the angle between the surface and the direction of fluid flow, or it may be increased by inserting an additional portion of the sensor member into the fluid flow, and decreased by Withdrawing a portion of the sensor member from the fluid flow. Both techniques are utilized in a preferred embodiment. In accordance with is another aspect of the invention, a fluid flow measuring device includes a conduit through which fluid may flow and a cantilevered beam having a free end portion thereof extending obliquely into the conduit to be deflected by the flow of fluid in the conduit. There is an arrangement which provides an indication of the amount of beam deflection and converts the indicated amount of beam deflection into a measure of the fluid flow rate. There may be a recess in the conduit forming a region of substantially reduced fluid flow and an end of the cantilevered beam which is opposite the free end is fixed within the recess. As beam deflection increases in response to increasing fluid flow, the portion of the beam within the recess increases and the portion of the beam exposed to the fluid flow decreases thereby providing enhanced flow sensitivity at lower flow rates and a measuring device of enhanced dynamic range. The present invention comprises a fluid flow measuring device comprising a conduit through which fluid may flow; a cantilevered beam having a free end portion thereof extending obliquely into the conduit to be deflected by the flow of fluid in the conduit; means responsive to beam deflection for providing an indication of the amount of beam deflection; and means for converting the indicated amount of beam deflection into a measure of the fluid flow rate.